Method of manufacturing glass substrate and method of manufacturing information recording medium

ABSTRACT

In a method of manufacturing a glass substrate for an information recording medium including a step for chemically strengthening the glass substrate by contacting the glass substrate with chemical strengthening processing liquid containing chemical strengthening salt, concentration of Fe and Cr is 500 ppb or less in said chemical strengthening salt, respectively. The concentration may be detected by the use of an ICP (Inductively Coupled Plasma) emission spectrometry analyzing method or a fluorescent X-ray spectroscopy analyzing method.

This is a divisional of application Ser. No. 09/540,886, filed Mar. 31,2000 now U.S. Pat. No. 6,523,367; the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to a method of manufacturing a magnetic recordingmedium used as a recording medium for an information processingapparatus and the like, and a method of manufacturing a substratethereof.

Recently, a magnetic disk has been used as an information recordingmedium. The magnetic disk is structured by forming a thin-film, such as,a magnetic layer, on a substrate. In this event, an aluminum substrateor a glass substrate has been generally used as the substrate.

However, the glass substrate, which can further narrow a space (namely,a flying height with respect to a magnetic head) between a magnetic headand a magnetic recording medium in comparison with the aluminumsubstrate, has been gradually replaced by the aluminum substrate inaccordance with high recording density in the recent years.

Such a glass substrate is generally manufactured by chemicallystrengthening to enhance strength and endure for impact when the glasssubstrate is mounted for a magnetic disk drive. Further, a surface ofthe glass substrate is polished with high accuracy so as to lower theflying height of the magnetic head to the utmost. Thereby, highrecording density has been realized.

In the meantime, a thin-film head has been recently replaced by amagneto-resistive head (namely, MR head) in the magnetic head to realizethe high recording density other than the glass substrate.

High surface flatness of the magnetic disk is required to realize a lowflying height necessary for the above-mentioned high recording density.In addition, when the MR head is used, the surface of the magneticrecording medium must have high flatness from the viewpoint of thermalasperity.

When the magnetic disk has projections on the surface of the magneticdisk, the MR head is affected by the projections to generate heat forthe MR head, and resistance value of the head is fluctuated to cause tooccur an error operation for electro-magnetic conversion by the heat.This phenomenon is defined as the above-mentioned thermal asperity.

Further, even when the surface of the magnetic disk has the highflatness, if the surface of the magnetic disk has the projections whichcause the thermal asperity, head crush brings about by the projections,and a magnetic film constituting the magnetic disk is peeled in thecause of the head crush. Thus, the projections give an adverse affectfor the magnetic disk.

Thus, demand has been gradually enhanced about the high surface flatnessof the magnetic disk to realize the low flying height and prevent thehead crush and the thermal asperity. The substrate surface having thehigh flatness is finally required to obtain the high surface flatness ofthe magnetic disk. However, the high recording density can be no longerrealized only by polishing the substrate surface with the high accuracy.

More specifically, even when the substrate surface is polished with thehigh accuracy, the high flatness can not be realized in case thatcontaminants are attached on the substrate. Although the contaminantshave been naturally and conventionally removed, the contaminants, whichhave been placed on the substrate and conventionally have beenpermitted, cause a problem in a recent level with respect to the highrecording density.

In this case, excessively small iron powder and stainless steel piece,which can not remove by the use of a normal washing process, areexemplified as this kind of contaminant. For example, it has beenconfirmed that when a chemical strengthening process is performed on thecondition that particles, such as, the iron powders are attached on theglass substrate or that the particles are attached on the glasssubstrate in the chemical strengthening processing liquid, irons arestrongly attached on the glass substrate to form island portions(namely, the projections) through oxidation reaction occurred in thechemical strengthening process and heat applied in the process.

It has been found out that when the thin-film, such as, the magneticfilm is laminated on the glass substrate, the island portions(projections) are formed on the surface of the magnetic disk to preventthe low flying height and to occur the head crush and the thermalasperity.

Therefore, investigation has been fully made about a cause in which suchfine iron powders are attached to the glass substrate. As a result, ithas been confirmed that the iron powders are contained in a chemicalstrengthening chamber for performing the chemical strengthening process,and in particular, a large number of iron powders are contained in achemical strengthening salt itself.

More specifically, when the number of the iron powders has beeninvestigated for each generation factor, the number of the iron powderscontained in the chemical strengthening salt itself before the chemicalstrengthening salt (sodium nitrate or potassium nitrate) is prepared tomake the chemical strengthening processing liquid is excessively high.

Further, it has been found out that the chemical strengthening saltitself contains the other particles which give an adverse affect for theinformation recording medium by attaching to the glass substrate for theinformation recording medium.

Meanwhile, disclosure has been made about a technique for removing theiron powders contained in atmosphere of the chemical strengtheningchamber for performing the chemical strengthening process and preventingthe iron powders from contaminating the chemical strengtheningprocessing liquid in Japanese Unexamined Patent Publication No.H10-194785.

Another disclosure has been made about a technique for removing the ironpowders contaminated from the atmosphere in the chemical strengtheningchamber into the chemical strengthening processing liquid by filteringthe chemical strengthening processing liquid by the use of a filterhaving superior corrosion resistance to high temperature, such as, amicrosieve (namely, wire cloth in which holes are opened by etching) inJapanese Unexamined Patent Publication No. H10-194786.

In this event, the former method is effective for removing the ironpowders contained in the atmosphere in the chemical strengtheningchamber for performing the chemical strengthening process.

Although the latter method has a constant effect, the number of the ironpowders contained the chemical strengthening salt itself before makingthe chemical strengthening processing liquid is excessively high asmentioned above, and as a result, the latter method is not sufficientlyeffective for removing the iron powders.

Further, the latter method is not enough to remove the other particleswhich attach to the glass substrate for the information recording mediumin the chemical strengthening process and give the adverse affect forthe information recording medium.

Moreover, the chemical strengthening process is carried out by replacingions contained in the glass by ions contained in original liquid for ionexchange, or distribution with respect to index of refraction isadjusted in a glass substrate for an electron device (including a glasssubstrate for a photomask, a glass substrate for a phase shift mask, ora glass substrate for an information recording medium, and hereinafter,will be used as the same meaning) or a glass substrate for an opticaldevice in addition to above-mentioned glass substrate for theinformation recording medium.

In such glass substrates, the original liquid for ion exchange containsFe and Cr, and thereby, the efficiency of ion exchange is lowered or theisland portions are formed. For example, the island portions shield alight beam, and as a result, a desired characteristic may not obtained.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a method ofmanufacturing a glass substrate for a magnetic recording medium which iscapable of effectively suppressing attachment of particles which attachto a glass substrate for an information recording medium in a chemicalstrengthening process and which give an adverse affect for aninformation recording medium.

In particular, it is another object of this invention to provide amethod of manufacturing a glass substrate for a magnetic recordingmedium which is capable of effectively suppressing formation ofprojections formed by attachment of fine iron powders to a glasssubstrate in a chemical strengthening process.

It is still another object of this invention to provide a method ofmanufacturing a magnetic recording medium which is capable ofeffectively suppressing attachment of particles which attach to a glasssubstrate for an information recording medium in a chemicalstrengthening process and which give an adverse affect for aninformation recording medium, and therefore, of obtaining an informationrecording medium having high quality and a slight of defects

It is further another object of this invention to provide a method ofmanufacturing a magnetic disk which is capable of realizing a low flyingheight and preventing head crush and thermal asperity.

It is still other object of this invention to provide a method ofmanufacturing an electron device or an optical device which is capableof effectively suppressing attachment of particles, which give anadverse affect by attaching to a glass substrate for an electron deviceor a glass substrate for an optical device in an ion exchange step, andtherefore, of having a slight of defects.

Inventors have proceeded research and development to achieve theabove-mentioned objects. As a result, it has been found out that islandportions (projections) are formed even when a size of an iron powder(including iron oxide or stainless steel) is 1 μm or less (for example,0.2 μm). Further, it is excessively effective to previously removeparticles, such as, the iron powders contained in chemical strengtheningsalt itself.

To this end, it has been found out that it is extremely effective toperform quantitative analysis of iron or chromium contained in thechemical strengthening salt, and thereby, this invention has beencompleted.

Namely, this invention has the following structures.

(Structure 1)

In a method of manufacturing a glass substrate for an informationrecording medium including a step for chemically strengthening the glasssubstrate by contacting the glass substrate with chemical strengtheningprocessing liquid containing a chemical strengthening salt,concentration of Fe and Cr is 500 ppb or less in said chemicalstrengthening salt, respectively. The concentration is detected by theuse of an ICP (Inductively Coupled Plasma) emission spectrometryanalyzing method or a fluorescent X-ray spectroscopy analyzing method.

(Structure 2)

In the method of the structure 1, the concentration of Fe and Cr is 100ppb or less in said chemical strengthening salt, respectively. Theconcentration is detected by the use of the ICP (Inductively CoupledPlasma) emission spectrometry analyzing method or the fluorescent X-rayspectroscopy analyzing method.

(Structure 3)

In the method of the structure 1, the concentration of Fe and Cr are 20ppb or less in said chemical strengthening salt, respectively. Theconcentration is detected by the use of the ICP (Inductively CoupledPlasma) emission spectrometry analyzing method or the fluorescent X-rayspectroscopy analyzing method.

(Structure 4)

In the method of either one of structures 1 through 3, quantitativeanalysis of element and concentration of impurity such as particles ofchemical strengthening of the chemical strengthening salt is carried outby the use of the an ICP (Inductively Coupled Plasma) emissionspectrometry analyzing method or the fluorescent X ray spectroscopyanalyzing method on the basis of a calibration curve. The calibrationcurve is determined by standard solutions each having differentconcentrations of Fe, Cr. The concentrations of Fe, Cr of chemicalstrengthening salt are determined by the calibration curve, afteranalyzing intensity of the atomic emissions of filtering solutiondissolved with the chemical strengthening salt in solvent by a filter,dissolving particles captured by the filter in acid, and measuring aknown concentration sample in advance.

(Structure 5)

In a method of manufacturing a glass substrate for an informationrecording medium including a step for chemically strengthening the glasssubstrate by contacting the glass substrate with chemical strengtheningprocessing liquid containing chemical strengthening salt, the number ofparticles having a particle diameter of 0.2 μm or more is 120000/g orless in the chemical strengthening salt. The number is determined from adifference between the particle number in a sample dissolved with thechemical strengthening salt in a reference liquid measured by the use ofa liquid particle counter and the particle number in the referenceliquid.

(Structure 6)

In a method of manufacturing a glass substrate for an informationrecording medium including a step for chemically strengthening the glasssubstrate by contacting the glass substrate with chemical strengtheningprocessing liquid containing chemical strengthening salt, the number ofparticles having a particle diameter of 0.2 μm or more is 900/mm² orless in the chemical strengthening salt when the particles contained inthe chemical strengthening salt of 1 g are captured by a filter having aminimum capturing particle diameter of 0.2 μm and a diameter of 13 mm incase that secondary electron image or back-scattered electron image isobserved by a scanning electron microscope (SEM) by the use of asecondary electron detector or a back-scattered electron image detector.

(Structure 7)

In a method of the structure 6, the number of the particles is 500/mm²or less in the chemical strengthening salt.

(Structure 8)

In a method of the structure 6, the number of the particles is 30/mm² orless in the chemical strengthening salt.

(Structure 9)

In a method of manufacturing a glass substrate for an informationrecording medium including a step for chemically strengthening the glasssubstrate by contacting the glass substrate with chemical strengtheningprocessing liquid containing chemical strengthening salt, the chemicalstrengthening salt does not become a black color or a gray color or ared brown color when the chemical strengthening salt is analyzed by theuse of colorimetric analysis.

(Structure 10)

In a method of manufacturing a glass substrate for an informationrecording medium including a step for chemically strengthening the glasssubstrate by contacting the glass substrate with chemical strengtheningprocessing liquid containing chemical strengthening salt, when solutiondissolved with said chemical strengthening salt in solvent is filteredby a filter, and particles are captured by the filter, a colorconcentration of the filter becomes a constant reference value or lessin the chemical strengthening salt.

(Structure 11)

In a method of inspecting chemical strengthening salt, the analyzingmethod of the structures 1 through 10 is used.

(Structure 12)

In a method of the structures 1 through 10, the glass substrate for theinformation recording medium comprises a glass substrate for a magneticdisk.

(Structure 13)

In a method of the structure 12, the glass substrate for the magneticdisk comprises a glass substrate for a magnetic disk which is used bycombining with a magneto-resistive head (MR head) or a giantmagneto-resistive head (GMR head).

(Structure 14)

In a method of manufacturing an information recording medium, at least arecording layer is formed on the glass substrate obtained by themanufacturing method of the glass substrate for the informationrecording medium of the structures 1 through 10.

(Structure 15)

In a method of manufacturing a magnetic disk, at least a magnetic layeris formed on the glass substrate obtained by the manufacturing method ofthe glass substrate for the information recording medium of thestructures 1 through 10.

(Structure 16)

In a method of manufacturing an optical glass substrate, including thestep of exchanging ions in a glass component with ions in an ionexchange processing liquid by dipping the glass component in the ionexchange processing liquid, a salt satisfying the conditions describedin any one of the structures 1 through 3 and 5 through 10 is used forthe ion exchange processing liquid.

(Structure 17)

In a method of the structure 16, the optical glass substrate is a glasssubstrate for an electronic device or a glass substrate for an opticaldevice and the ion exchange processing liquid is a chemicalstrengthening processing liquid containing a chemical strengtheningsalt.

(Structure 18)

In a method of manufacturing a glass substrate for an informationrecording medium including a chemical strengthening step forstrengthening the glass substrate by replacing a part of first ionscontained in the glass substrate by second ions in processing liquidhaving an ion diameter larger than the first ion by contacting the glasssubstrate with chemical strengthening processing liquid containing achemical strengthening salt, content of particles is suppressed in orderto prevent generation of thermal asperity in the chemical strengtheningsalt. The particles cause the thermal asperity.

(Structure 19)

In a method of manufacturing a glass substrate for an informationrecording medium including a chemical strengthening step forstrengthening the glass substrate by replacing a part of first ionscontained in the glass substrate by second ions in a processing liquidhaving an ion diameter larger than the first ion by contacting the glasssubstrate with chemical strengthening processing liquid containingchemical strengthening salt, the number of particles having a particlediameter of 0.2 μm or more contained in the chemical strengthening saltis 12000/ml or less.

(Structure 20)

In a method of the structure 19, a ratio of the particles having theparticle diameter of 0.2 μm or more is 25% or less in the chemicalstrengthening salt.

(Structure 21)

In a method of the structure 19 or 20, the number of particles having aparticle diameter of 0.2 μm or more contained the chemical strengtheningsalt is 4000/ml or less.

(Structure 22)

In a method of the structures 18 through 21, the particle contains iron.

(Structure 23)

In a method of the structures 18 through 22, the glass substrate for theinformation recording medium comprises a glass substrate for a magneticdisk.

(Structure 24)

In a method of the structure 23, the glass substrate for the magneticdisk comprises a glass substrate for a magnetic disk which is used bycombining with a magneto-resistive head (MR head) or a giantmagneto-resistive head (GMR head).

(Structure 25)

In a method of manufacturing an information recording medium, at least arecording layer is formed on the glass substrate obtained by themanufacturing method of the glass substrate for the informationrecording medium of the structures 18 through 24.

(Structure 26)

In a method of manufacturing an magnetic disk, at least a magnetic layeris formed on the glass substrate obtained by the manufacturing method ofthe glass substrate for the information recording medium of thestructures 18 through 24.

(Structure 27)

A method of manufacturing a glass substrate for a magnetic disk,comprising the steps of:

preparing a glass substrate;

judging whether or not the amount of particles contained in a chemicalstrengthening salt itself is not greater than a predetermined referencevalue;

reducing, if the predetermined reference value is exceeded, the amountof particles contained in the chemical strengthening salt itself to alevel not greater than the predetermined reference value;

preparing a chemical strengthening processing liquid by mixing thechemical strengthening salt containing the particles in an amount notgreater than the predetermined reference value; and

chemically strengthening the glass substrate by replacing a part offirst ions contained in the glass substrate by second ions contained inthe processing liquid and having an ion diameter larger than that of thefirst ions by contacting the glass substrate with the chemicalstrengthening processing liquid.

(Structure 28)

In the method of the structure 27, the predetermined reference value isdetermined by preliminarily obtaining correlation between the amount ofparticles contained in the chemical strengthening salt itself and aglide height, selecting, with reference to the correlation, a particularamount of particles corresponding to a desired glide height, and settingthe particular amount as the predetermined reference value.

(Structure 29)

In a method of manufacturing a magnetic disk, at least a magnetic layeris formed on a principal surface of the glass substrate for the magneticdisk of the structure 27 or 28.

According to the structures 1-3, the concentration of Fe and Cr is 500ppb or less in the chemical strengthening salt, respectively. Theconcentration is detected by the use of an ICP (Inductively coupledPlasma) emission spectrometry analyzing method or a fluorescent X-rayspectroscopy analyzing method.

Consequently, formation of an island portion can be effectivelysuppressed. In this case, fine iron powder in chemical strengtheningprocessing liquid is attached to the glass substrate, and thereby, theisland portion is formed. Therefore, a low flying height can beachieved, and the head crush and the thermal asperity can be preventedin the magnetic disk.

When the concentration of Fe and Cr exceeds 500 ppb, a ratio, in whichthe island portion is formed, becomes excessively high, and the heightof the island portion and the density of the island portions become highduring the chemical strengthening step. Further, the faulty rate becomeshigh in the glide test of the 1.2μ inch height, and the probability ofreproduction error due to the thermal asperity also becomes high.

From the same view, the concentration of Fe and Cr is preferably 250 ppbor less, and more preferably, 100 ppb or less, 20 ppb or less, 10 ppb orless, 5 ppb or less, and 1 ppb or less.

In the ICP (Inductively Coupled Plasma) emission spectrometry analyzingmethod, elements to be analyzed and contained in the sample arevaporized and excited by inductively coupled plasma generated byinductively coupling high frequency power, and the quantitative analysisis carried out by measuring the obtained emission intensity in an atomspectrum line (JIS K 0116).

According to the structure 4, quantitative analysis of element andconcentration of the chemical strengthening salt is carried out by theuse of the an ICP (Inductively Coupled Plasma) emission spectrometryanalyzing method or the fluorescent X-ray spectroscopy analyzing methodon the basis of a calibration curve. The calibration curve is determinedby filtering solution dissolved with the chemical strengthening salt insolvent by a filter, dissolving particles captured by the filter inacid, and measuring a known concentration sample in advance.

Consequently, the metal based particles, such as, iron or chromium, aredissolved in acid, and the quantitative analysis can be performed by theICP (Inductively Coupled Plasma) emission spectrometry analyzing methodhaving high sensitivity.

According to the structure 5, the particle number is determined from thedifference between the particle number in the sample dissolved with thechemical strengthening salt in the reference liquid and the particlenumber in the reference liquid (namely, blank).

Thereby, an accurate measuring value of the particle number having aslight of variation can be obtained, and an accurate judgement ispossible on the basis of the measuring value.

Further, the number of particles having a particle diameter of 0.2 μm ormore is 120000/g or less in the chemical strengthening salt. Inconsequence, the attachment of the particles, which give an adverseaffect for the information recording medium by attaching to the glasssubstrate for the information recording medium in the chemicalstrengthening processing liquid, can be effectively suppressed.

In particular, fine iron powder in the chemical strengthening processingliquid attach to the glass substrate. In this case, the formation of theisland portions can be effectively suppressed.

In this event, the particle has a particle diameter of 0.2 μm or more.This is because the particles having the particle diameter of notexceeding 0.2 μm do not give an affect for the formation of the islandportion which causes the thermal asperity.

When the number of particles having a particle diameter of 0.2 μm ormore exceeds 120000/g, the fine iron powder in the chemicalstrengthening processing liquid attaches to the glass substrate, and aratio, in which the island portion is formed, becomes high.Consequently, the number of the island portions becomes high, and theheight and density of the island portion also becomes high. Further,this is not preferable because a ratio, which the thermal asperity andthe head crush occur, becomes high.

Similarly, when the number of particles having a particle diameter of0.2 μm or more exceeds 120000/g, the attachment number of the particles,which give an adverse affect for the information recording medium byattaching to the glass substrate for the information recording medium inthe chemical strengthening processing liquid, becomes high. This is notpreferable.

From the same viewpoint, the number of particles having a particlediameter of 0.2 μm or more contained in the chemical strengthening saltis preferably 8000/g or less, and more preferably, 4000/g or less. Thisreason is explained as follows. Namely, the number of the particlescontained in the chemical strengthening salt is directly reflected forthe generation of the island portion in the chemical strengtheningprocessing liquid and the attachment of the particles. Therefore, theprobability for generating the island portions or the attachment numberof the particles can be reduced by reducing the number of the particlescontained in the chemical strengthening salt.

Further, the density of the island portions is desirably 0.002/mm² orless, and more desirably, 0.0003/mm² or less.

According to the structures 6˜8, when secondary electron image orback-scattered electron image is observed by a scanning electronmicroscope (SEM) by the use of a secondary electron detector or aback-scattered electron detector, the number of particles (such as ironpowder, stainless steel piece) having a particle diameter of 0.2 μm ormore is 900/mm² or less in the chemical strengthening salt when theparticles contained in the chemical strengthening salt of 1 g arecaptured by a filter having a minimum capturing particle diameter of 0.2μm and a diameter of 13 mm.

Thereby, the fine iron powder in the chemical strengthening processingliquid attaches to the glass substrate and the formation of the islandportion can be effectively suppressed. Consequently, the low flyingheight can be achieved, and the head crush and the thermal asperity canbe prevented in the magnetic disk.

When the number of the particles (such as iron powder and stainlesssteel piece) exceeds 500/mm2, a ratio, in which the island portion isformed, becomes excessively high, and the height and density of theisland portion also become high. Further, the faulty rate becomes highin the glide test of the 1.2μ inch height, and the probability of thereproduction error due to the thermal asperity also becomes high. Thenumber of the particles (such as the iron powder and stainless steelpiece) is preferably 500/mm² or less, 300/mm² or less, 100/mm² or less,30/mm² or less, and more preferably, 10/mm² or less.

According to the structure 9, the chemical strengthening salt does notbecome a black color or a gray color or a red brown color when thechemical strengthening salt is analyzed by the use of colorimetricanalysis.

Thereby, the fine iron powder in the chemical strengthening processingliquid attaches to the glass substrate and the formation of the islandportion can be effectively suppressed. Consequently, the low flyingheight can be achieved, and the head crush and the thermal asperity canbe prevented in the magnetic disk.

When the chemical strengthening salt becomes the black color or the graycolor or the red brown color in case that the chemical strengtheningsalt is analyzed by the use of the colorimetric analysis, a ratio, inwhich the island portion is formed during the chemical strengtheningstep, becomes excessively high, and the height and density of the islandportion also become high. Further, the faulty rate becomes high in theglide test of the 1.2μ inch height, and the probability of thereproduction error due to the thermal asperity also becomes high.

According to the structure 10, the solution of the chemicalstrengthening salt is filtered by the filter, particles are captured bythe filter, and the color concentration of the filter becomes a constantreference value or less in the chemical strengthening salt.

Thereby, the fine iron powder in the chemical strengthening processingliquid attaches to the glass substrate and the formation of the islandportion can be effectively suppressed. Consequently, the low flyingheight can be achieved, and the head crush and the thermal asperity canbe prevented in the magnetic disk.

When the color concentration of the filter is denser than the constantreference value, a ratio, in which the island portion is formed duringthe chemical strengthening step, becomes excessively high, and theheight and density of the island portion also become high. Further, thefaulty rate becomes high in the glide test of the 1.2μ inch height, andthe probability of the reproduction error due to the thermal asperityalso becomes high.

According to the structure 11, the analyzing method of the structures 1through 10 is used as the inspecting method of the chemicalstrengthening salt.

According to the structure 12, the glass substrate for the informationrecording medium is a glass substrate for a magnetic disk. Thereby, theformation of the island portion, which causes the head crush, can beeffectively suppressed. Therefore, the low flying height can berealized, and the head crush and the thermal asperity can be preventedin the magnetic disk.

According to the structure 13, the glass substrate for the magnetic diskmay be a glass substrate for a magnetic disk which is used by combiningwith a magneto-resistive head. Thereby, the formation of the islandportion, which causes the thermal asperity and the head crush, can beeffectively suppressed. As a result, the low flying height can berealized. When the magneto-resistive head is used, this is particularlyeffective because the low flying height is required.

According to the structure 14, the glass substrate obtained by themanufacturing method of the glass substrate for the informationrecording medium of the structures 1 through 10 is used. Thereby, theattachment of the particles, which give an adverse affect for theinformation recording medium by attaching to the glass substrate for theinformation recording medium in the chemical strengthening processingliquid, can be effectively suppressed. Further, the informationrecording medium having high quality and a slight of defects can beobtained.

According to the structure 15, the glass substrate obtained by themanufacturing method of the glass substrate for the informationrecording medium of the structures 1 through 10 is used.

Thereby, the fine iron powder in the chemical strengthening processingliquid attaches to the glass substrate and the formation of the islandportion can be effectively suppressed. Consequently, the low flyingheight can be achieved, and the head crush and the thermal asperity canbe prevented in the magnetic disk.

According to the structure 16-17, in a method of manufacturing anoptical glass substrate, including the step of exchanging ions in aglass component with ions in an ion exchange processing liquid bydipping the glass component in the ion exchange processing liquid, asalt satisfying the conditions described in any one of the structures 1through 3 and 5 through 10 is used for the ion exchange processingliquid. The optical glass substrate is a glass substrate for anelectronic device or a glass substrate for an optical device and the ionexchange processing liquid is a chemical strengthening processing liquidcontaining a chemical strengthening salt.

Consequently, the island portion is not formed on the chemicallystrengthened glass substrate or the glass substrate in whichdistribution with respect to index of refraction is adjusted, and theelectron device or the optical device having high quality can beobtained.

According to the structure 16, the original liquid for the ion exchangemay be a chemical strengthening processing liquid containing chemicalstrengthening salt. This is effective to fabricate the chemicallystrengthened glass (used in the electron device or the optical device).Furthermore, the reduction of the efficiency of the ion leakage can besuppressed/

According to the structure 18, content of particles, which are containedin the chemical strengthening salt itself and cause the thermalasperity, is suppressed.

Thereby, the fine iron powder in the chemical strengthening processingliquid attached to the glass substrate and the formation of the islandportion can be effectively suppressed. Consequently, the low flyingheight can be achieved, and the head crush and the thermal asperity canbe prevented in the magnetic disk.

According to the structure 19, the number of particles having a particlediameter of 0.2 μm or more is 12000/ml or less in the chemicalstrengthening salt. In consequence, the attachment of the particles,which give an adverse affect for the information recording medium byattaching to the glass substrate for the information recording medium inthe chemical strengthening processing liquid, can be effectivelysuppressed.

In particular, fine iron powder in the chemical strengthening processingliquid attaches to the glass substrate. The formation of the islandportions can be effectively suppressed.

In this case, the particle has a particle diameter of 0.2 μm or more.This is because the particles having the particle diameter of notexceeding 0.2 μm do not give an affect for the formation of the islandportion which causes the thermal asperity.

When the number of particles having a particle diameter of 0.2 μm ormore exceeds 12000/ml, the fine iron powder in the chemicalstrengthening processing liquid attaches to the glass substrate, and aratio, in which the island portion is formed, becomes high.Consequently, the number of the island portions also becomes high, andthe height and density of the island portion also becomes high. Further,this is not preferable because a ratio, which the thermal asperity andthe head crush occur, becomes high.

Similarly, when the number of particles having a particle diameter of0.2 μm or more exceeds 12000/ml, the attachment number of the particles,which give an adverse affect for the information recording medium byattaching to the glass substrate for the information recording medium inthe chemical strengthening processing liquid, becomes high. This is notdesirable.

From the same viewpoint, the number of particles having a particlediameter of 0.2 μm or more contained in the chemical strengthening saltis preferably 8000/ml or less, and more preferably, 4000/ml or less.This reason is explained as follows. Namely, the number of the particlescontained in the chemical strengthening salt is directly reflected forthe generation of the island portion in the chemical strengtheningprocessing liquid and the attachment of the particles. Therefore, theprobability for generating the island portions or the attachment numberof the particles can be reduced by reducing the number of the particlescontained in the chemical strengthening salt.

Further, the density of the island portions is desirably 0.002/mm² orless, and more desirably, 0.0003/mm² or less.

In this case, the number of the particles contained in the chemicalstrengthening salt was measured by the following predetermined method.

Namely, the chemical strengthening salt (the potassium nitrate and thesodium nitrate) was dissolved in the super pure water of 90 ml with 10g, respectively. Herein, it is to be noted that the super pure watermeans water which is sufficiently cleaned and does not contain theparticles that give an affect for the measurement.

The solution was successively measured three times by the particlecounter (made by Lion Ltd. or PMS Ltd.) for the liquid with 5 ml.

Further, the number of the particles (the total of the particle numberin each particle size) contained per 1 ml was determined and converted,and these average values were decided as the particle number. In thiscase, the total of the particle number corresponds to the total of thenumber of the particles which exist in a range of each particle sizewhich is arbitrarily determined.

According to the structure 20, a ratio of the particles, which give alarge affect for the formation the island portion (projection) and havea large particle diameter (specifically, the particle diameter of 0.2 μmor more) is 25% or less in the chemical strengthening salt. Thereby, theformation of the island portion (projection) can be effectivelyprevented. From the same viewpoint, the ratio is preferably 20% or less,and more preferably, 15% or less.

According to the structure 21, the number of particles having a particlediameter of 2 μm or more contained said chemical strengthening salt is4000/ml or less. Thereby, the formation of the island portion(projection) can be further effectively prevented.

This reason is explained as follows. Namely, the particle having theparticle diameter of 2 μm or more gives a strong affect in comparisonwith the particle having not exceeding 2 μm. Further, the particlediameter of the particle of iron, whcih causes the thermal asperity, isabout 2 μm or more.

According to the structure 22, the particle contains a particle of iron.When the particle contained in the chemical strengthening salt wasanalyzed, O, Na, Mg, Al, Si, Cl, Fe, Cr and the like were detected. Inparticular, when the particle is Fe (iron), and the particle (iron) isattached to the glass substrate in the chemical strengthening processingliquid, the island portion (projection) dissolved with iron is formed onthe glass substrate by the oxidation reaction occurred in the chemicalstrengthening process and the heat applied in this process. These islandportions cause the thermal asperity and the head crush with highprobability.

Meanwhile, when the particle has a relatively large size, the particlesare dissolved to form the island portions (projections). In themeantime, the particle has a relatively small size, the particlesaggregate and dissolve to form the island portions (projections). Suchphenomenon has been confirmed by a microscope.

Therefore, a remarkable effect particularly appears by controllingquantity (the number) of the particles, such as, the iron contained inthe chemical strengthening salt in the magnetic disk.

Further, the particle of the iron contains the iron oxide and SUS inaddition to the iron. Moreover, metal, such as, Cr and Al is exemplifiedas material of the particle for forming the island portion (projection)by the above-mentioned oxidation reaction and the heat.

According to the structure 23, when the glass substrate for theinformation recording medium is a glass substrate for a magnetic disk,the formation of the island portion, which causes the head crush, can beeffectively suppressed. In consequence, the low flying height can beachieved and the head crush can be prevented.

According to the structure 24, when the glass substrate for the magneticdisk is a glass substrate for a magnetic disk which is used by combiningwith a magneto-resistive head, the formation of the island portion,which causes the thermal asperity, can be effectively suppressed. As aresult, the low flying height can be realized. When themagneto-resistive head is used, this is particularly effective becausethe low flying height is required.

According to the structure 26, the glass substrate obtained by themanufacturing method of the glass substrate for the informationrecording medium of the structures 18 through 24 is used.

Thereby, the fine iron powder in the chemical strengthening processingliquid attaches to the glass substrate and the formation of the islandportion can be effectively suppressed. Consequently, the low flyingheight can be achieved, and the head crush and the thermal asperity canbe prevented in the magnetic disk.

According to the structure 27 and 28, a correlation between a content ofparticles contained in chemical strengthening salt itself and a glideheight is determined in advance. The content of the particles, whichbecomes a desired glide characteristic, is determined as a referencesetting value from the correlation. It is judged that the content of theparticles contained in the chemical strengthening salt itself is thepredetermined reference setting value or less. The content of theparticles contained in the chemical strengthening salt itself is set tothe predetermined setting value or less when the content exceeds thereference setting value.

Thereby, chemical strengthening processing liquid can be made bypreparing the chemical strengthening salt. Therefore, the generation ofthe island portion or the attachment of the particles can be reduced inthe chemical strengthening process.

In this case, the reference setting value can be selected in accordancewith a permitted level of defect required for the information recordingmedium. Further, the reference setting value is determined such that amagnetic head is arranged in opposition to a principal surface of amagnetic disk (or a glass susbstrate), the magnetic disk is relativelymoved for the magnetic disk (or the glass substrate) with apredetermined height, and a desired glide height characteristic isobtained.

In other words, the reference setting value is determined on the basisof the result of the glide test, and thereby, the head crush or thethermal asperity can be effectively prevented. Herein, it is to be notedthat the desired glide characteristic may mean that generating rate ofhit and crush becomes 0% in the glide height of 1.2μ inch or less.

The reference value in the structure 27 may be determined by obtainingcorrelation between the amount of particles contained in the chemicalstrengthening salt itself and a glide height, selecting, with referenceto the correlation, a particular amount of particles corresponding to adesired glide height, and setting the particular amount as the referencevalue, as described in the structure 28. Alternatively, the referencevalue may be determined by obtaining the correlation between the amountof particles and the occurrence ratio of the thermal asperity, thecorrelation between the amount of particles and the defect ratio in theglide test, the correlation between the amount of particles and theheight and the density of the island portion, and so on, selecting aparticular amount of particles contained in the chemical strengtheningsalt itself so that the magnetic disk has desired characteristics, andsetting the particular amount as the reference value.

According to the structure 29, at least a magnetic layer is formed on aprincipal surface of the glass substrate for the magnetic disk of thestructure 27 or 28. Thereby, the low flying height can be achieved, andthe head crush and the thermal asperity can be prevented in the magneticdisk.

In this case, the particle in this invention contains a particle ofiron. When the particle contained in the chemical strengthening salt wasanalyzed, O, Na, Mg, Al, Si, Cl, Fe, Cr and the like were detected. Inparticular, when the particle is Fe (iron), the particle (iron) isattached to the glass substrate in the chemical strengthening processingliquid. In this case, the particles are strongly attached onto the glasssubstrate by the oxidation reaction occurred in the chemicalstrengthening process and the heat applied in this process to form theisland portions (projections). These island portions cause the thermalasperity and the head crush with high probability.

Meanwhile, when the particle has a relatively large size, the particleis strongly attached to form the island portions (projections). In themeantime, the particle has a relatively small size, the particlesaggregate and are strongly attached to form the island portion(projection). Such phenomenon has been confirmed by a microscope.

Therefore, a remarkable effect particularly appears by controllingquantity of the particles, such as, the iron contained in the chemicalstrengthening salt in the magnetic disk.

Further, the particle of the iron contains the iron oxide and thestainless steel in addition to the iron. Moreover, Ti, Al, Cl, Ce, and aglass piece are exemplified as the other material of the particle forforming the island portion (projection) by the above-mentioned oxidationreaction and the heat.

Subsequently, description will be made about the method of manufacturingthe glass substrate for the information recording medium.

In this invention, quantity (the number) and concentration of theparticles, such as, the iron contained in the chemical strengtheningsalt itself are analyzed by the above-mentioned analyzing method, andthe chemical strengthening salt satisfying a reference value or less isused. This is a feature of this invention.

Thus, it is judged by the analysis that the quantity of the particlescontained in the chemical strengthening salt itself falls within therange of the predetermined reference value or less. As a result of thejudgement, the chemical strengthening salt of the reference value orless is prepared, and thereby, the chemical strengthening processingliquid can be made such that the quantity of the particles falls withinthe range the predetermined value or less. Consequently, generation ofthe island portions and the attachment of the particles can be reducedin the chemical strengthening processing step. Herein, it is to be notedthat the reference value can be set in accordance with a permitted levelof the defects required for the information recording medium.

For example, the above-mentioned reference value is determined in thefollowing manner. Namely, the magnetic head is arranged in opposite to aprincipal surface of the magnetic disk (or the glass substrate), and themagnetic head is relatively moved for the magnetic disk (or the glasssubstrate) with a predetermined glide height to obtain a desired glidecharacteristic.

In other words, the head crush and the thermal asperity can beeffectively prevented by determining the reference value on the basis ofthe result of the glide test. For example, the desired glidecharacteristic means that the glide height is 1.2μ inch or less, andgenerating rate of hit or crush becomes 0%.

For instance, the particles are removed by the use of capturing means,such as, a filter on the condition that the chemical strengthening saltis dissolved in water to remove the particles contained in the chemicalstrengthening salt itself on the basis of the above-mentioned analysisresult.

The quantity of the particles contained in the chemical strengtheningsalt can be controlled to the desired quantity or concentration byselecting performance (minimum capturing particle size) or the kind ofthe filter.

For example, the desired quantity means that the content of theparticles of the particle diameter of 0.2 μm or more is 12000/ml orless, or 4000/ml or less.

Further, a plurality of filters having different minimum capturingparticle sizes are used. Thereby, after the particle having a largeparticle size is removed by the filter having a large minimum capturingparticle size, the particle having a small particle size is removed bythe filter having a small minimum capturing particle size.

It is unnecessary to use reagent refined with high purity as allcomponents except for the chemical strengthening salt in this invention.In particular, the chemical strengthening salt, which is removed andcleaned only particles that gives an adverse affect for the iron powderor the information recording medium, is used, and thereby, the cost isreduced.

Naturally, although such reagent refined with high purity can be used,the cost is inevitably increased. Further, it is preferable that thechemical strengthening salt used in this invention contains no additionagent for preventing consolidation. This is because the addition agentfor preventing the consolidation may contain a large number ofparticles.

Low temperature type chemical strengthening is preferable as thechemical strengthening method. In such low temperature type chemicalstrengthening, ion exchange is carried out within a region not exceedinga glass transition temperature. Potassium nitrate, sodium nitrate, ornitrate salt mixed them, potassium sulfate, sodium sulfate, or sulfatesalt mixed them, or NaBr, KBr and salt mixed them can be used as alkalisolvent salt used in the chemical strengthening processing liquid.

In this case, aluminosilicate glass, soda-lime glass, and borosilicateglass are exemplified as the glass substrate. A glass substrate for amagnetic recording medium, a glass substrate for an optical recordingmedium, and a glass substrate for an electro-optical recording mediumare exemplified as the glass substrate for the information recordingmedium. In particular, this invention achieves a remarkable effect withrespect to the magnetic disk for the magneto-resistive head and thesubstrate thereof.

Subsequently, description will be made about a method of manufacturingthe magnetic recording medium (the magnetic disk) according to thisinvention.

In this invention, the magnetic recording medium is manufactured byforming at least a magnetic layer on the above glass substrate for themagnetic recording medium according to this invention.

According to this invention, the attachment of the particles, whichcause the thermal asperity or the head crush, can be effectivelysuppressed. In consequence, when the magnetic recording medium havingthe magnetic layer on the glass substrate is fabricated, it is difficultto form the island portions formed by the particles, which cause thethermal asperity, on the principal surface of the glass substrate.Thereby, the thermal asperity or the head crush can be prevented withhigher level.

For example, the low glide height of the 1.2μ inch or less can be alsorealized because the island portions are not formed. In particular, themagnetic recording medium, which reproduces by the magneto-resistivehead, can sufficiently achieve function as the magneto-resistive head.Further, the magnetic recording medium can sufficiently achieveperformance thereof as the magnetic recording medium of CoPt base andthe like because it can be suitably used for the magneto-resistive head.

Moreover, no defect occurs for a film, such as, the magnetic layer bythe particles which cause the thermal asperity. As a result, no errordue to the defect takes place.

The magnetic recording medium generally has the predetermined flatnessand surface roughness, and is manufactured by successively laminating anunderlying layer, an magnetic layer, a protection layer, and a lubricantlayer on the glass substrate for the magnetic disk in which the chemicalstrengthening process is performed for the surface as needed.

The underlying layer is selected in relation to the magnetic layer. Atleast one metal selected from a group consisting of nonmagnetic metals,Cr, Mo, Ta, Ti, W, V, B, Al, Ni may be used as a material of theunderlying layer (including a seed layer). The metal Cr or the Cr alloyis preferably used as the material of the underlying layer to enhance amagnetic characteristic where the magnetic layer includes Co as a maincomponent. In addition, the underlying layer may not always be formed bya single layer but may be formed by a multi-layer composed of aplurality of identical or different layers. For example, deposition maybe made as the underlying layer formed by the multi-layer, such as,Cr/Cr, Cr/CrMo, Cr/CrV, NiAl/Cr, NiAl/CrMo, and NiAl/CrV.

In this invention, no limitations are imposed as to the magnetic layeralso.

The magnetic layer may be, for example, a layer which contains Co as amain component and which has a composition selected from CoPt, CoCr,CoNi, CoNiCr, CoCrTa, CoPtCr, CoNiPt, CoNiCrPt, CoNiCrTa, CoCrTaPt,CoCrPtB, and CoCrPtSiO. In addition, the magnetic layer has amulti-layer structure (for example, CoPtCr/CrMo/CoPtCr,CoCrTaPt/CrMo/CoCrTaPt). Such a structure is obtained by dividing amagnetic film by a nonmagnetic film (for example, Cr, CrMo, CrV) toreduce a noise, as known in the art. The magnetic layer for themagneto-resistive head (MR head) or the giant magneto-resistive head(GMR head) contains impurity elements selected from a group consistingof Y, Si, rare-earth elements, Hf, Ge, Sn and Zn, oxides of theseimpurity elements in addition to the Co-based alloy.

Further, the magnetic layer may have a granular structure whereinmagnetic grains, such as Fe, Co, FeCo and CoNiPt, are dispersed in thenonmagnetic film comprising a ferrite-based material, an iron-rareearth-based material, SiO₂, and BN. Further, the magnetic layer may havea recording form of either an in-plane magnetization type or aperpendicular magnetization type.

No restrictions are imposed as to the protection layer also according tothis invention. Specifically, the protection layer may be formed by achromium film, a chromium alloy film, a carbon film, a zirconia film anda silica film and may be successively deposited on the glass substratetogether with the underlying layer and the magnetic layer by the use ofthe known in-line sputtering apparatus. The protection layer may beformed by a single layer or a multi-layer including a plurality oflayers of an identical material or different materials.

In addition, the other protection layer, such as a SiO₂ film, may beused instead of the above protection layer. Such a SiO₂ film may beformed on the chromium film by dispersing colloidal silica fine grainsin tetraalkoxysilane diluted with an alcohol-based solvent andthereafter by coating and baking the dispersed grains.

Moreover, the lubricating layer is not restricted to the above. Forexample, the lubricating layer is formed by diluting perfluoropolyether(PFPE) with a solvent, such as freon-based solvent, and applying it onthe medium surface by a dipping method, a spin coating method, or aspraying method, and firing the medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a filter holder used in anembodiment according to this invention;

FIG. 2 is a SEM picture showing an analysis result of chemicalstrengthening salt according to the other embodiment of this invention;

FIG. 3 is a SEM picture showing an analysis result of chemicalstrengthening salt according to the other embodiment of this invention;

FIG. 4 is a SEM picture showing an analysis result of chemicalstrengthening salt according to a comparative example; and

FIG. 5 is a SEM picture showing an analysis result of chemicalstrengthening salt according to the other comparative example.

DESCRIPTION OF PREFERRED EMBODIMENT

Hereinafter, detail description will be made about this invention withexamples.

EXAMPLE 1 AND COMPARATIVE EXAMPLE 1

(1) Roughing Step

First, a sheet glass of aluminosilicate is formed by a down draw methodand is cut by a grinding wheel into a disc-shaped glass substrate whichhas a diameter of 96 mm and a thickness of 3 mm. The glass substrate islapped or polished to a diameter of 96 mm and a thickness of 1.5 mm by arelatively rough diamond grindstone.

In this case, the disc-shaped glass substrate may be formed by directlypressing a melting glass with a cope, a drag and a drum instead of thedown draw method. Alternatively, the glass substrate may be formed bythe use of a float method.

In this event, the above-mentioned glass which is chemicallystrengthened contains, by mol %, 57-74% of SiO₂, 0-2.8% of ZnO₂, 3-15%of Al₂O₃, 7-16% of Li₂O, and 4-14% of Na₂O as main components.

For example, the above-mentioned composition can be expressed in termsof a mol representation and comprises, by mol %, 67% of SiO₂, 1.0% ofZnO₂, 9.0% of Al₂O₃, 12.0% of Li₂O and 10.0% of Na₂O.

Subsequently, the both principal surfaces of the glass substrate areground or lapped by a diamond grindstone having grains smaller thanthose of the above grindstone. In this case, a load was set to theextent of 100 Kg. Thereby, the principal surface 3 of the glasssubstrate 1 was ground into a surface roughness Rmax (measured by JIS B0601) of about 10 μm.

Next, an opening was formed at a center portion of the glass substrateby using the cylindrical grindstone. Further, the outer side end surfaceis ground to a diameter of 95 mm. Thereafter, the outer and inner sideend surfaces were chamfered. In this case, the side end wall of theglass substrate had a surface roughness Rmax of about 4 μm.

(2) Lapping Step

Subsequently, the lapping step was performed for the glass substrate toimprove dimension and shape accuracy. The lapping step is carried out byusing a lapping apparatus. In this case, the lapping step is conductedtwo times by changing grain degrees from #400 to #1000.

Specifically, the lapping was performed for the both principal surfacesof the glass substrate so that the principal surfaces had a surfaceaccuracy of 0-1 μm and the surface roughness Rmax of about 6 μm. In thisevent, the lapping was carried out by rotating an inner gear and anouter gear by the use of alumina grains having a grain degree of #400 onthe condition that the load L was kept at about 100 Kg.

Next, the lapping is performed by changing the grain degree of thealumina grain into #1000. As a result, the surface roughness Rmaxbecomes about 2 μm. Subsequently, the glass substrate 1 was immersed inwashing units by using natural detergent and water to be washed.

(3) Mirror Finishing Step of the Side End Surface

The end surface of the glass substrate was polished by the use of abrush by rotating the glass substrate so that the end surface had thesurface roughness Rmax of 1 μm and the surface roughness Ra of 0.3 μm.Next, the glass substrate was washed with water after the mirror finishprocess of the end surface.

(4) First Polishing Step

Next, first polishing was performed by a polishing apparatus to remove adefect and a distortion remaining in the above lapping process.Specifically, a hard polisher (which may be a cerium impregnated foamedurethane pad, such as MHC15 made by Speedfam) was used as a polisher andthe first polishing was performed the following polishing condition.

Polishing liquid: oxide cerium+water

Load: 300 g/cm² (L=238 kg)

Polishing time: 15 minutes

Removing amount: 30 μm

Revolution of lower surface plate: 40 rpm

Revolution of upper surface plate: 35 rpm

Revolution of inner gear: 14 rpm

Revolution of outer gear: 29 rpm

The glass substrate 1 was washed by being successively dipped in washingunits of natural detergent, pure water, IPA (isopropyl alcohol), IPA(vapor drying) after the first polishing.

(5) Second Polishing Step

Next, second polishing was conducted by changing the above hard polisherinto a soft polisher (which may be a polishing pad of a suede type, suchas Politex made by Speedfam) by using the polishing apparatus used inthe first polishing process. The polishing condition is similar to thefirst polishing step except for the load of 100 g/cm², the polishingtime of 5 minutes and the removing amount of 5 μm. The glass substrate 1was immersed in washing units of the natural detergent, the pure water,the IPA (isopropyl alcohol), the IPA (vapor drying) to be washed thereinafter the second polishing step. In this case, a supersonic wave wasapplied to each of the washing units.

(6) Preparation and Analysis Step of Chemical Strengthening Salt

Subsequently, a part of each row material sack (lot) of potassiumnitrate and sodium nitrate as the raw material of the chemicalstrengthening newly obtained was extracted, and samples were made atevery lots as follows.

First, the particles contained in the chemical strengthening salt itselfwere removed, and the samples of potassium nitrate and sodium nitrate,which were sufficiently cleaned, were fabricated, respectively (example1). In this case, the cleanness was performed by dissolving thepotassium nitrate and sodium nitrate in super pure water, respectively,and filtering by the use of a filter for liquid.

Further, samples of potassium nitrate and sodium nitrate, which wereextracted from each row material sack and not cleaned at all, werefabricated, respectively (comparative example 1).

Fe and Cr contained in the potassium nitrate and sodium nitrate weremeasured by the use of the ICP (Inductively Coupled Plasma) emissionspectrometry analysis as follows. The chemical strengthening saltincluded a variety of particles. In this case, attention was paid formetal based particles (such as iron powder, stainless steel) in whichremoval due to attachment is difficult and which causes defects, and theanalysis was carried out. Therefore, all used jigs were made by glass orplastic to prevent dust generation of the particles from the used jigsor fitter holders.

{circle around (1)} A part of the potassium nitrate and sodium nitratewere extracted or obtained from the samples (example 1), which weresufficiently cleaned, and the samples (comparative example 1), whichwere not cleaned at all, and about 200 g was weighed after a dry processwas carried out during day and night at 110° C. The samples waredissolved in hot water made by heating the super pure water, and theobtained solution was filtered by the use of a membrane filter (maximumcapturing particle diameter: 0.2 μm) made by PTFE(polytetrafluoroethylene) in which a hydropholic process was performed.This membrane filter was dipped in acid water solution (hydrochloricacid, nitric acid, sulfuric acid, hydrofluoric acid, acid mixture ofthem) containing aquaregia (hydrochloric acid 3: nitric acid 1) of 2 ml,and recovering substances captured on the filter were dissolved. Purewater was added for the obtained acid water solution, and was set to 50ml.

{circle around (2)} The analysis was conducted by the use of the ICP(Inductively Coupled Plasma) emission spectrometry analysis (SPS-1200 VRmade by Seiko Instruments Inc.).

First, solutions of concentration of known four points were preparedwith respect to Fe and Cr, and these elements were quantified by theabsolute calibration curve by the use of the above-mentioned ICP(Inductively Coupled Plasma) emission spectrometry analysis. Themeasuring condition is represent as follows.

TABLE 1 Element Fe Cr Sample Dilution Ratio 40 g/50 ml Standard Solution(ppb) 5, 50, 500, 5000 Wavelength (nm) 259.940 267.716 BG Correction 0/0Mode P D Integration Time   1 sec Number of Times 3 of Integration RFPower  1.8 kW Measurement Height 11.6 mm

{circle around (3)} The elements were quantified by the use of theabove-mentioned ICP (Inductively Coupled Plasma) emission spectrometryanalysis by using the samples obtained in the above {circle around (1)}under the same measuring condition as the above case. Measuringconcentration was determined from the calibration curve obtained in theabove {circle around (2)}. Herein, it is to be noted that conversionfrom the measuring concentration to the sample concentration wasdetermined from the following relation equation (1).Content in the sample [ppb]=ICP measuring value [mg/1]×10⁹[ppb]×flaskcapacity [ml]/1000[ml/l]/sample weight [mg]  (1)

As a result of the measurement, the content was 500 ppb or less in theexample 1 while the content was exceeding 500 ppb in the comparativeexample 1. The result is represented by Table 2. In this case, it hasbeen confirmed that variation exists between the lots.

TABLE 2 KNO3 NaNO3 Fe Cr Fe Cr Example 1 18.9 1.8 4.1 0.5 Comparative566.9 45.5 577.2 39.4 Example 1 (ppb)

(6′) Preparation Step of Chemical Strengthening Processing Liquid

The particles contained in the chemical strengthening salt itself wereremoved, and the samples of potassium nitrate and sodium nitrate, whichwere sufficiently cleaned, were prepared, respectively.

Specifically, the potassium nitrate and sodium nitrate were dissolved insuper pure water, respectively, and the particles contained in thechemical strengthening salt itself were removed by the use of a filterfor liquid.

The particles contained in the potassium nitrate and the sodium nitratewere measured by a particle counter for liquid, respectively. In detail,the potassium nitrate and the sodium nitrate were dissolved in the superpure water of 90 ml with 10 g, respectively. Herein, it is to be notedthat the super pure water means water which is sufficiently cleaned anddoes not contain the particles that give an affect for the measurement.

The solution was successively measured three times by the particlecounter for the liquid with 5 ml. In this case, the particle counter ismade by Lion Ltd. in the case of the particle diameter of 0.2 μm or moreand not exceeding 2.0 μm while the particle counter is made by PMS Ltd.in the case of the particle diameter of 2.0 μm or more.

Further, the number of the particles (the total of the particle numberin each particle size) contained per 1 ml was determined and converted,and these average values were decided as the particle number.

As a result, the number of the particles contained in the sodium nitratewas higher than the number of the particles contained in the potassiumnitrate. The number of the particles contained in the sodium nitrate was1912/ml in the particle diameter of 0.2 μm or more and not exceeding 0.3μm, 1192/ml in the particle diameter of 0.3 μm or more and not exceeding0.5 μm, 161/ml in the particle diameter of 0.5 μm or more and notexceeding 1.0 μm, 10/ml in the particle diameter of 1.0 μm or more andnot exceeding 2.0 μm, 310/ml in the particle diameter of 2.0 μm or moreand not exceeding 3.0 μm, 111/ml in the particle diameter of 3.0 μm ormore and not exceeding 4.0 μm, 60/ml in the particle diameter of 4.0 μmor more and not exceeding 5.0 μm, 36/ml in the particle diameter of 5.0μm or more and not exceeding 6.0 μm, 169/ml in the particle diameter of6.0 μm or more, 686/ml in the particle diameter of 2.0 μm or more, and3275/ml in the particle diameter of 0.2 μm or more and not exceeding 2.0μm.

The total number of the particles having the particle diameter of 0.2 μmor more was 3961/ml, and the ratio of the particles having the particlediameter of 0.2 μm or more was 17.3%.

(7) Chemical Strengthening Step

Chemical strengthening processing liquid was obtained by mixing anddissolving potassium nitrate and sodium nitrate with 60% and 40% (total:73.5 kg) by using the chemical strengthening salt analyzed in theabove-mentioned step of (6) or (6′). The obtained chemical strengtheningprocessing liquid was heated up to 400° C., and the glass substrate,which was washed and preheated to 300° C., was dipped in the chemicalstrengthening liquid solution for 3 hours.

The chemical strengthening step was carried out so that the entiresurface of the glass substrate was chemically strengthened with aplurality of glass substrates retained at the end surface in a holder.

Under the circumstances, lithium ions and sodium ions on a surface layerof the glass substrate were replaced by sodium ions and potassium ionsin the chemical strengthening processing liquid by dipping each glasssubstrate in the chemical strengthening processing liquid. Thus, theglass substrate is chemically strengthened.

A compressive stress layer formed in the surface layer of the glasssubstrate had a thickness of about 100-200 μm. Next, the chemicallystrengthened glass substrate was dipped in a water tank of 20° C.,quickly cooled and retained for 10 minutes.

Subsequently, the cooled glass substrate was dipped in a sulfuric acidheated up to 40° C., and was washed by the supersonic wave.

The surface roughness Ra of the glass substrate obtained via theabove-mentioned process falls within the range between 0.5 nm and 1 nm.

The glass surface was observed by the use of the optical microscope. Asa result, the island portion, which causes the thermal asperity and thehead crush, was not detected in the example 1. On the other hand, theisland portion, which causes the thermal asperity and the head crush,was detected in the comparative example 1.

(8) Magnetic Disk Manufacturing Step

A seed layer (a film thickness of 40 nm) of NiAl (Ni:50 at %, Al:50 at%), an underlying layer (a film thickness: 25 nm) of CrMo (Cr:94 at %,Mo:6 at %), a magnetic layer (a film thickness:27 nm) of CoCrPtTa (Co:75at %, Cr:17 at %, Pt:5 at %, Ta:3 at %), and a hydrogenation carbonprotection layer (a film thickness: 10 nm) were successively depositedon the both surfaces of the glass substrate for the magnetic diskobtained in the above-mentioned step by using the known in-linesputtering apparatus. Further, a liquid lubricant material layer (a filmthickness: 1 nm) made by perfluoropolyether was formed on the protectionlayer by the use of the dip method to obtain the magnetic disk for theMR head.

(Evaluation)

A glide test (the glide height: 1.2μ inch, peripheral speed: 8 m/s)(1500 samples) was performed in connection with the obtained magneticdisk. As a result of the test, hit (the head contacts with theprojection on the surface of the magnetic disk) and crash (the headcollides with the projection on the surface of the magnetic disk) didnot occur in the example 1. Further, it has been confirmed that nodefect occurred for the film, such as, the magnetic layer by theparticles which cause the thermal asperity.

In addition, a reproduction test was performed for the magnetic disk ofthis example after the glide test by the use of the magneto-resistivetype head. As a result of the test, it has been confirmed that noreproduction errors due to the thermal asperity take place in connectionwith the samples (500 samples).

The glass substrate for the magnetic disk and the magnetic disk werefabricated in the same manner as the example 1 except that the chemicalstrengthening salt, which was not cleaned, was used to make the chemicalstrengthening processing liquid, and the same evaluation was carried outin the comparative example 1.

The concentration of Fe and Cr contained the chemical strengthening saltexceeded 500 ppb in the comparative example in which the chemicalstrengthening was performed by the chemical strengthening processingliquid which was not cleaned. During the chemical strengthening step,ratio, in which the island portion was formed, became excessively high,and a height and a destiny of the island portion became high. Further,faulty ratio was high in the glide test (5000 samples) of 1.2μ inchheight. Moreover, although the reproducing test (500 samples) wasperformed, the probability of the reproduction error due to the thermalasperity was also high.

EXAMPLE 2-5

Subsequently, the glass substrate was fabricated in the same manner asthe example 1 except that a several kinds of chemical strengtheningsalts, in which contents of Fe and Cr were adjusted, were prepared in arefining process of the salt. In this case, the content of Fe and Crcontained each salt was measured in the same manner as the example 1.

EXAMPLE 2

-   -   KNO3:3.4 ppb (Fe), 0.5 ppb (Cr)    -   NaNO3:3.3 ppb (Fe), 0.8 ppb (Cr)

EXAMPLE 3

-   -   KNO3:74.2 ppb (Fe), 12.5 ppb (Cr)    -   NaNO3:13.0 ppb (Fe), 3.3 ppb (Cr)

EXAMPLE 4

-   -   KNO3:195.5 ppb (Fe), 10.3 ppb (Cr)    -   NaNO3:36.2 ppb (Fe), 5.7 ppb (Cr)

EXAMPLE 5

-   -   KNO3:478.2 ppb (Fe), 22.6 ppb (Cr)    -   NaNO3:87.9 ppb (Fe), 45.5 ppb (Cr)

When the glass surface of the obtained glass substrate was observed bythe use of the optical microscope, the island portion, which caused thethermal asperity or the head crush, was not detected. It has beenconfirmed that the height or the generation ratio or the density of theisland portion becomes higher as the content of Fe or Cr contained inthe chemical strengthening salt (KNO3, NaNO3) is higher. However, theheight of the island portion was smaller than the height of the islandportion in the comparative example 1, and did not cause the thermalasperity and the head crush.

EXAMPLE 6 AND COMPARATIVE EXAMPLE 2

The glass substrate for the magnetic disk and the magnetic disk werefabricated in the same manner as the example 1 and the comparativeexample 1 except that the analysis of the chemical strengthening saltwas carried out by the use of the SEM, and the same evaluation wasperformed. In this case, the analysis was carried out as follows.

{circle around (1)} The chemical strengthening salt included a varietyof particles. Attention was paid for the metal based particles (such asiron powder, stainless steel piece), which the removal due to theattachment is difficult and cause the defects, and the analysis wascarried out. Therefore, all used jigs were made by glass or plastic toprevent dust generation of the particles from the used jig or filterholder.

The used jig was washed in advance by using pure water in a clean room.A vial bottle was attached with neutral detergent in BEMCOT (made byNabelin Co., Ltd.), and an internal, a spout, and a cap of the bottlewere washed twice to remove the detergent.

Thereafter, a membrane filter unit of 0.2 μm was attached to aninjector, and the filter holder (including the membrane filter having adiameter of 13 mm and a minimum capturing diameter of 0.2 μm) wasmounted at the tip thereof, and the internal of the vial bottle wassuitably rinsed in filtered pure water (hereinafter, referred to as purewater). A pincette made by ceramic was suitably rinsed at the tip edgein predetermined pure water. The filter holder was suitably rinsed inthe predetermined pure water.

In this time, an attached spacer for fixing the filter was fitted to thefilter holder, and were rinsed together. The jig after washing wasplaced on a clean BEMCOT, and attention was paid such that the particleswere not attached thereto again.

As illustrated in FIG. 1, a spacer 2 and a membrane filter 1 are setbetween washed filter holders A and B by using the pincette made byceramic. Herein, it is to be note that the membrane filter 1 has adiameter of 13 mm and a minimum capturing particle diameter of 0.2 μm.In this event, the holder is tightly fixed such that liquid is notleaked from between the filter holders A and B.

{circle around (2)} A side of the filter holder was mounted to theinjector (without an injector needle) of 50 ml, and the samples (salt200 g, potassium nitrate cleanness, non-cleanness, solution) areinjected in the injector. In this case, after the samples are sucked bythe injector (without the injector needle), the samples may be attachedto the A side of the filter holder. Herein, the injector needle was madeby metal, and was not used because the dust generation might beoccurred.

The samples were injected in the filter by pushing a piston of theinjector, and a flittering process was carried out. In this event, thepiston was not pushed in one breath, and the piston was pushed such thatextracted liquid was dropped. Further, when bubbles exist inside thefilter holder A, a large pushing force is necessary, and nonuniformityappears in obtaining contaminants. Therefore, presence and absence ofthe bubbles was confirmed.

After the filtering process, the predetermined pure water of 10 ml pourthree times and flushing to remove potassium and sodium from the top ofthe filter. When they are remained or left with large quantity on thefilter, the SEM observation becomes difficult by charge-up, and anaccurate analysis can not be performed during the EDX analysis (energydispersion type X ray spectroscopy analysis) because they disturb.

Under this circumstance, when the hot pure water is used, the flushingcan be readily conducted. In this case, the filter is inserted into adesiccator containing drying material for about 24 hours to removewater.

{circle around (3)} The obtained membrane filter is placed on a carbonsample stand, and an edge is formed by the use of carbondortite. In thiscase, the dortite is not applied for an entire periphery, and a space ofabout 2 mm is formed. Thereby, air entered between the membrane filterand a both surface tape can be removed in vacuum removal duringvaporizing described later. The vaporizing is performed by the use ofplatinum palladium for 30 seconds with 10 mA to obtain the sample forthe SEM analysis. In consequence, the observation and the analysis canbe carried out with an acceleration voltage of 15 kV with no problem.

The SEM observation was performed by the use of the obtained sample forthe SEM analysis. As the condition thereof, the observation was made bythe use of the back-scattered electron image detector or the secondaryelectron detector with a voltage of 15 kV and WD of 15 mm. In this case,when the SEM apparatus having the back-scattered electron image detectorwas used, the filter containing much carbon was reflected with blackwhile metal or glass clearly reflects with white by an atom numbereffect in which an object can more clearly viewed as the atom number ishigher. Consequently, the SEM apparatus is extremely convenient forvisual observation or picture judgement and counting accuracy is alsoenhanced. In the case of secondary electron image, it is difficult todistinguish the filter from the contaminant.

The stage is moved to a location in which the particles are remarkable,and an entire image (magnification of 100 times) and an enlarged image(magnification of 500 times) were pictured, respectively. In this case,FIG. 2 shows the entire image (magnification of 100 times) about KNO₃ inthe example 2, and FIG. 3 shows the enlarged image (magnification of 500times). FIG. 4 shows the entire image (magnification of 100 times) aboutKNO₃ in the comparative example 2, and FIG. 5 shows the enlarged image(magnification of 500 times). From these pictures, the quantity of theparticles can be determined per a unit area. In this event, when anentire screen is charged-up by the particles during the observation, theprocess is again started from the initial step.

Successively, the EDX (Energy Dispersion type X ray spectroscopy method)analysis (ESCA (Electron Spectroscopy for Chemical Analysis), SIMS(Secondary Electron Mass Analyzing method) may be used) is carried out.As the condition thereof, the EDX analysis was preformed by the use ofthe back-scattered electron image detector with acceleration voltage of15 kV and WD of 15 mm. Element analysis was carried out by randomlyselecting the particles of 10 points from a viewing field ofmagnification of 500 times. Thereby, element, which constitutes theparticle contained in the chemical strengthening salt, is judged.

{circle around (4)} As a result of the measurement, the quantity of theparticles (such as iron powder and stainless steel piece) having theparticle diameter of 0.2 μm or more was 30 samples/mm² or less (25samples/mm² or less) in the example 2. In the meantime, the quantity ofthe particles (such as iron powder and stainless steel piece) having theparticle diameter of 0.2 μm or more was more than 900 samples/mm² (915samples/mm²) in the comparative example 2.

(Evaluation)

When the glass surface before the magnetic film was deposited wasobserved by the use of the microscope, the island portion, which causedthe thermal asperity or the head crush was not detected in the example 2while the island portion, which caused the thermal asperity or the headcrush was detected in the comparative example 2.

A glide test (the glide height: 1.2μ inch, peripheral speed: 8 m/s)(1500 samples) was performed in connection with the obtained magneticdisk. As a result of the test, the hit and the crash did not occur inthe second example. Further, it has been confirmed that no defectoccurred for the film, such as, the magnetic layer by the particleswhich cause the thermal asperity.

In addition, a reproduction test was performed for the magnetic disk ofthis example after the glide test by the use of the magneto-resistivetype head. As a result of the test, it has been confirmed that noreproduction errors due to the thermal asperity take place in connectionwith the samples (500 samples).

On the other hand, the faulty rate was high (10%) in the glide test(5000 samples) of the 2μ inch height in the comparative example 2, andwhen the reproduction test (500 samples) was performed, the probabilityof the reproduction error due to the thermal asperity was also high.

EXAMPLES 7-9

Subsequently, the glass substrate was fabricated in the same manner asthe example 7 except that a several kinds of chemical strengtheningsalts, in which contents of Fe and Cr were adjusted, were prepared in arefining process of the salt. In this case, the contents of Fe and Crcontained each salt were measured in the same manner as the example 7.

EXAMPLE 7

-   -   87 samples/mm² (particles of 0.2 μm or more)

EXAMPLE 8

-   -   292 samples/mm² (particles of 0.2 μm or more)

EXAMPLE 9

-   -   485 samples/mm² (particles of 0.2 μm or more)

When the glass surface of the obtained glass substrate was observed bythe use of the optical microscope, the island portion, which caused thethermal asperity or the head crush, was not detected. It has beenconfirmed that the height, the generation ratio, and the density of theisland portion became higher as the content of Fe or Cr contained in thechemical strengthening salt (KNO3, NaNO3) was higher. However, theheight of the island portion was smaller than the height of the islandportion in the comparative example 1, and did not cause the thermalasperity and the head crush.

EXAMPLE 10 AND COMPARATIVE EXAMPLE 3

In the example 6 and the comparative example 2, the filter after theflittering may be visually observed, compared a color and correlationbetween the color and the above SEM analysis result may be obtained.Thereby, the quality of the chemical strengthening salt can be judged,and therefore, this is an effective means.

For example, the membrane filter was taken out from the filter holder,and water was completely removed by the use of Kimwipe (made by NabelinCo., Ltd.). In this event, when crystal of the salt was visuallydetected on the membrane filter, the process was again started from theinitial step because the flushing might be not sufficiently performed.

Next, only the membrane filter was pinched by a transparent adhesivesheet (for example, made by Mieko Ltd.: Pouch and the like), and wastightly covered. This is because the color is changed later when themembrane filter is tightly covered with a paper.

Subsequently, the membrane filter, which was tightly covered, was placedon a white paper, and was tightly covered again. The membrane filter wascolored to a brown-based color, and therefore, the white paper was usedas an underlying paper.

As a result of the correlation with the above SEM analysis result,defective product occurred when the color after the filtering was deeperthan a constant reference level. In the case of almost no color or lightcolor, no defective product occurred.

EXAMPLE 11 AND COMPARATIVE EXAMPLE 4

The glass substrate for the magnetic disk and the magnetic disk werefabricated in the same manner as the example 1 and the comparativeexample 1 except that the analysis of the chemical strengthening saltwas carried out by the use of colorimetric analysis, and the similarevaluation was performed. In this event, the analysis was carried out asfollows.

In such colorimetric analysis, it is necessary to use color except forblack color, the gray color, and the red brown color when the chemicalstrengthening salt is analyzed by the colorimetric analysis. Thecolorimetric analysis was carried out by resolving 200 g salt in asolution and filtering the solution by the use of a filter having afilter size of 13 mmφ and a mesh size of 0.2 μm.

As a result a correlation with the above IPC analysis result, the blackcolor and the gray color corresponds to stainless steel, and the redbrown color corresponds to rust (iron oxide). When these colors appearthe stainless steel or iron oxide is contained with 500 ppb or more inconcentration and defective product occurred. In the case of almost nocolor and light facing color, no defective product occurred.

EXAMPLE 12

The glass substrate for the magnetic disk and the magnetic disk werefabricated in the same manner as the example 1 and the comparativeexample 1 except that the analysis of the chemical strengthening saltwas carried out by the use of the particle counter, and the similarevaluation was performed. In this event, the analysis is carried out asfollows.

{circle around (1)} The chemical strengthening salt (newly obtainedpotassium nitrate) of 5.000 g was entered into a clean vial bottle tomake samples, and super pure water pf 50 ml, which was filtered by thefilter of 0.2 μm, was added, and was covered. This vial bottle wasdipped in hot water of 50° C. to dissolve the chemical strengtheningsalt. In this case, it has been confirmed that the chemicalstrengthening salt was not recrystallized when the temperature of thesolution was lowered to the room temperature.

{circle around (2)} In the meantime, the optical dispersion typeautomatic particle counter (made by RION Ltd.: KL-20 type) is used asthe particle counter in liquid, and IPA (isopropyl alcohol) of 10 mlpoured from a sample injection inlet of the particle counter five times,and was flushed.

Successively, reference liquid (background) was prepared in a washedbeaker. In this case, super pure water of 60 ml, which was filtered bythe filter of 0.2 μm, was silently entered. Further, this referenceliquid of 10 ml poured five times as the same manner as the above caseto completely discharge IPA.

{circle around (3)} Successively, the reference liquid of 10 ml poursfive times in the same manner as the above case, and, the number of theparticles is countered, and the particle number in the reference liquidis determined at every particle diameter from an average value of latethree times. In this case, when the count of 0.2 μm is 200 or more inthe count of the later three times, or when the count of total fivetimes is 300 or more, the reference liquid is again made to perform thecount again.

{circle around (4)} The solution of 1.0 ml of the chemical strengtheningsalt manufactured in {circle around (1)} is added to the referenceliquid of 500 ml, and is slowly agitated. The sample pours five times inthe same manner as the above case, and the count of the particle numberis carried out, and the particle number in the sample liquid isdetermined at every particle diameter from an average value of the laterthree times.

{circle around (5)} The increasing number of the particle number of 10ml is determined from the following equation.

The increasing number of the particle number=the particle number in thesample liquid−the particle number in the reference liquid.

The particle number of impurity in the chemical strengthening salt of 1g is determined from the following equation.The increasing number of the particle number in 10 ml/[(5 g/50 ml)×(1ml/500 ml)×10 ml]

As a result, the number of the particles contained in the sodium nitratewas higher than the number of the particles contained in the potassiumnitrate. The number of the particles contained in the sodium nitrate was1912/g in the particle diameter of 0.2 μm or more and not exceeding 0.3μm, 1192/g in the particle diameter of 0.3 μm or more and not exceeding0.5 μm, 161/g in the particle diameter of 0.5 μm or more and notexceeding 1.0 μm, 10/g in the particle diameter of 1.0 μm or more andnot exceeding 2.0 μm, 310/g in the particle diameter of 2.0 μm or moreand not exceeding 3.0 μm, 111/g in the particle diameter of 3.0 μm ormore and not exceeding 4.0 μm, 60/g in the particle diameter of 4.0 μmor more and not exceeding 5.0 μm, 36/g in the particle diameter of 5.0μm or more and not exceeding 6.0 μm, 169/g in the particle diameter of0.6 μm or more, 686/g in the particle diameter of 2.0 μm or more, and3275/g in the particle diameter of 0.2 μm or more and not exceeding 2.0μm.

The total number of the particles having the particle diameter of 0.2 μmor more was 3961/g, and the ratio of the particles having the particlediameter of 0.2 μm or more was 17.3%.

(Evaluation)

When the glass surface before the magnetic film was deposited by the useof the optical microscope, the island portions, which cause the thermalseparate and the head crush, did not detected.

A glide test (the glide height: 1.2μ inch, peripheral speed: 8 m/s)(1500 samples) was performed in connection with the obtained magneticdisk. As a result of the test, the hit and the crash did not occur.Further, it has been confirmed that no defect occurred for the film,such as, the magnetic layer by the particles, which cause the thermalasperity.

In addition, a reproduction test was performed for the magnetic disk ofthis example after the glide test by the use of the magneto-resistivetype head. As a result of the test, it has been confirmed that noreproduction errors due to the thermal asperity take place in connectionwith the total of a plurality of samples (500 samples).

EXAMPLES 13-15 AND COMPARATIVE EXAMPLE 5

Subsequently, the glass substrate for the magnetic disk and the magneticdisk were fabricated in the same manner as the example 11 except thatthe chemical strengthening salts different in the number of theparticles were prepared to make the chemical strengthening processingliquid, and the same evaluation was carried out. In this case, thechemical strengthening salt, which was not cleaned, was used in thecomparative example.

Table 3 shows the number of the particles (the particle diameter of 2 μmor more, the particle diameter of 0.2 μm or more and not exceeding 2.0μ,these total) of the chemical strengthening salt (sodium nitrate), theheight of the island portion (average value), the density of the islandportions (average value), and the result (faulty rate) (5000 samples) ofthe glide test of the magnetic disk.

In the examples 6˜8 and the comparative example 5, the ratios of theparticles of 2.0 μm or more among the total particles having thediameter of 0.2 μm or more were 18.2 (example 12), 15.0% (example 13),22.1% (example 14), and 25.3% (comparative example 5).

TABLE 3 Compara- Example Example Example Example tive Evaluation Item 56 7 8 Example 5 Number particles ≧ 686 273 1089 26450 48790 of 2.0 μmparticles/g 0.2 ≦ particles 3275 1227 6167 93360 144410 <2.0 μm Total3961 1500 7256 119810 193200 Island Height no island no island 12 nm 20nm 100 nm Portion (Average) Density no island no island 0.0003/mm²0.002/mm² 0.007/mm² (Average) Glide Test (1.2 μinch) 0% 0% 0.2% 0.5% 10%

Apparent from Table 3, the cleaned chemical strengthening salt was used(as a result, the number of the particles contained in the chemicalstrengthening processing liquid was reduced). Thereby, it has been foundout that the island potions, which were formed on the surface of theglass substrate, could be effectively reduced, and further, the resultof the reproduction test due to the magneto-resistive head was excellentin the glide test.

In particular, the island portion was not be formed when the number ofthe particles having the particle diameter of 0.2 μm or more was 4000/gor less (the number of the particles having the particle diameter of 2.0μm or more was 700/g or less). Therefore, it is preferable because nodefect takes place in the glide test.

In the meantime, when the number of the particles having the particlediameter of 0.2 μm or more contained in the chemical strengthening saltexceeds 120000/g, in particular, when the ratio of particles of theparticle diameter of 2.0 μm or more exceeds 25%, the ratio of theformation of the island portion becomes high during the chemicalstrengthening step, the height of the island portion exceeds 30 nm, andthe density of the island portions exceeds 0.002/mm².

In consequence, the faulty rate (the ratio of the hit or the head crush)was high in the glide test of the 1.2μ inch height. Further, althoughthe reproduction test (500 samples) was performed, the probability ofthe reproduction error due to the thermal asperity became higher as thefaulty rate of the glide test was also higher.

Meanwhile, the number of the particles having the particle diameter of0.2 μm or more contained in the chemical strengthening salt was 193200/gin the comparative example 5 in which the chemical strengthening wasperformed in the chemical strengthening processing liquid which was notcleaned.

During the chemical strengthening step, the ratio of the formation ofthe island portion becomes excessively high, the height of the islandportion was 100 nm, and the density of the island portions was0.0027/mm².

Further, the faulty rate was high (10%) in the glide test (5000 samples)of the 1.2μ inch height. Moreover, although the reproduction test (500samples) was performed, the probability of the reproduction error due tothe thermal asperity was high.

When the island portions formed in the examples 14-15 and thecomparative example were analyzed and observed, the component of theisland portion contained iron. Further, the island portion, in which asolid-like particle was attached, and the island portion, in which theparticle (iron) was strongly attached by the oxidation reaction in thechemical strengthening step and the applied heat, were observed.

Further, the quantify of the particles contained in the chemicalstrengthening salt is previously determined as a reference setting valueso as to obtain a desired glide characteristic on the basis of theresult of the glide test described in Table 3. Thereby, it has beenfound out that the glass substrate having the desired glidecharacteristic for the magnetic disk, and the magnetic disk having atleast the magnetic layer on the glass substrate can be manufactured.

For example, the chemical strengthening processing salt (solution salt)is obtained by using the chemical strengthening salt, in which thenumber of the particles having the particle diameter of 0.2 μm or moreis 4000/g or less as the quantify of the particles contained in thechemical strengthening salt itself before making the chemicalstrengthening processing liquid, and the chemical strengthening processis carried out by the use of the chemical strengthening processingliquid. Thereby, the chemically strengthened glass substrate for themagnetic disk, which satisfies the glide height of 1.2μ inch, can beobtained.

EXAMPLE 15

In the manner similar to that mentioned in conjunction with the example1, a glass substrate for a magnetic disk and a magnetic disk wereproduced and subjected to the similar evaluation except that the ICPemission spectroscopy ({circle around (2)} in the example 1) wasreplaced by the X-ray fluorescence analysis using a total-reflectionX-ray fluorescence spectrometer. Hereinafter, the method and theconditions of the X-ray fluorescence analysis will be described.

The X-ray fluorescence analysis comprises the steps of irradiatingintense primary X-rays obtained by an X-ray tube onto a sample togenerate characteristic X-rays, measuring the wavelength and theintensity of the characteristic X-rays, identifying elements containedin the sample, preparing an analytical or calibration curve consideringthe matrix effect by the use of a standard reagent to determine thequantities of the elements contained in the sample.

Generally, the X rays are incident at an incident angle between 30° and90°. However, the incident angle may be between a critical angle and30°. The critical angle is a maximum incident angle exhibiting the totalreflection of the incident X-rays. In the latter case, the sensitivityis high in principle. More preferably, the incident angle is on theorder of ¼ to ½ of the critical angle.

In this example, the analysis was carried out by the use of the totalreflection X-ray fluorescence spectrometer (TREX610 manufactured andsold by Technos Co., Ltd.). The measurement condition was as follows.

Target tungsten (W) (other targets may be used) Analyzing Crystalmonochromator Detector Si(Li) SSD Atmosphere vacuum Voltage  30 kVCurrent 200 mA Measurement Time 500 sec Incident Angle 0.05°

At first, for each of Fe and Cr, solutions having four levels ofconcentrations were prepared. By the use of the above-mentioned X-rayfluorescence spectrometer, quantitative determination of these elementswas carried out in accordance with an absolute analytical curvetechnique under the above-mentioned conditions.

Measurement is carried out as follows. A predetermined amount of theabove-mentioned solution is dropped onto a clean silicon wafer anddried. The primary X-rays are simultaneously irradiated throughout thedry residue to generate the characteristic X-rays of each of Fe and Cr.The intensities of the characteristic X-rays are measured. It is notedhere that the residue may be in the liquid phase instead of the dryphase.

The result of quantitative determination was substantially equal to thatobtained in the ICP emission spectroscopy in the example 1. In addition,no projections causing the thermal asperity or the head crush wereobserved. No reproduction error was caused by the gliding defect of themagnetic disk or the thermal asperity.

Although the glide height of 1.2μ inch is exemplified in theabove-mentioned examples, the invention is not limited to the height. Asmentioned before, the correlation between the glide height and thequantity of the particles contained in the chemical strengthening saltitself is determined in advance, the quantity of the particles containedin the chemical strengthening salt itself may be controlled so as toobtain the desired glide height.

Although this invention has been explained with the preferred examples,this invention is not limited to the above-mentioned examples.

For example, the filter size of and the mesh size of the filter used inthe analysis are not restricted to those mentioned above. In dependenceupon the analyzing method, the filter size is appropriately selectedbetween 10 and 50 mmφ and the mesh size is appropriately selectedbetween 0.1 and 2 μm.

For example, even when the fluorescent X-ray spectroscopy analyzingmethod is used instead of the ICP (Inductively Coupled Plasma) emissionspectrometry analyzing method in the example 1, the quantitativeanalysis can be carried out in the same manner as the above case.Herein, it is to be noted that the ICP (Inductively Coupled Plasma)emission spectrometry analyzing method is superior in sensitivity andrepeatability.

Further, reliability of the analysis can be enhanced by combining aplurality of analyzing methods, and product quality can be enhanced byhigh analyzing accuracy.

In addition, this invention can be applicable as long as the glasssubstrate is manufactured for use in the information recording mediumthrough the chemical strengthening step, and can be widely applied for avariety of glass substrates for the information recording medium, suchas, the glass substrate for the optical disk and the glass substrate forthe magneto-optical disk. In this event, when the density of the islandportions is high in the glass substrate for optical disk and the glasssubstrate for magneto-optical disk, an adverse affect is given forrecord and reproduction. Therefore, the quantity of the particlescontained in the chemical strengthening salt itself is preferablycontrolled by taking the density of the island portions intoconsideration.

As mentioned above, the quantity of the particles contained in thechemical strengthening salt itself is analyzed, only the chemicalstrengthening salt, which passes the analysis, is used in thisinvention. In consequence, the attachment of the particles, which givesan adverse affect for the information recording medium by attaching theglass substrate for the information recording medium in the chemicalstrengthening processing liquid, can be effectively suppressed.

In particular, the formation of the island portions, which cause thethermal asperity or the head crush, can be effectively suppressed. Inthis case, the fine iron powder in the chemical strengthening processingliquid and the like attaches to the glass substrate, and thereby, theisland portions are formed.

As a result, the information recording medium having high quality and aslight of defects can be obtained, and in particular, the low flyingheight can be achieved, the head crush can be prevented, and thereduction of the reproducing function due to the thermal asperity can beprevented in the magnetic disk. Further, the low flying height of 1.2μinch or less can be realized.

1. A method of manufacturing a glass substrate for a magnetic diskincluding a step for chemically strengthening the glass substrate bycontacting the glass substrate with chemical strengthening processingliquid containing chemical strengthening salt, wherein the chemicalstrengthening salt itself contains particles comprising Fe and Cr, themethod comprises the steps of: preparing a cleaned chemicalstrengthening salt comprising Fe and Cr, each of the Fe and Cr having aconcentration of 500 ppb or less, wherein the concentration of 500 ppbor less is detected by analyzing a solution obtained by dissolvingparticles of Fe and Cr captured by a filter into acid by the use of anICP (Inductively Coupled Plasma) emission spectrometry analyzing methodor a fluorescent X-ray spectroscopy analyzing method, and said capturedparticles are obtained by filtering a solution in which the chemicalstrengthening salt is dissolved in solvent using the filter, the filterhaving a minimum capturing particle diameter of 0.2 μm, producing thechemical strengthening processing liquid by the use of the cleanedchemical strengthening salt and, performing a chemical strengtheningtreatment by contacting the glass substrate with heated chemicalstrengthening processing liquid.
 2. A method as claimed in claim 1,wherein: the concentration of Fe and Cr is 100 ppb or less in saidchemical strengthening salt, respectively, the concentration beingdetected by the use of the ICP (Inductively Coupled Plasma) emissionspectrometry analyzing method or the fluorescent X-ray spectroscopyanalyzing method.
 3. A method as claimed in claim 1, wherein: theconcentration of Fe and Cr is 20 ppb or less in said chemicalstrengthening salt, respectively, the concentration being detected bythe use of the ICP (Inductively Coupled Plasma) emission spectrometryanalyzing method or the fluorescent X-ray spectrscopy analyzing method.4. A method as claimed in claim 1, wherein: quantitative analysis ofelement and concentration of impurity such as particles of chemicalstrengthening of the chemical strengthening salt is carried out by theuse of the ICP (Inductively Coupled Plasma) emission spectrometryanalyzing method or the fluorescent X-ray spectroscopy analyzing methodon the basis of a calibration curve, the calibration curve beingdetermined by standard solutions each having different concentrations ofFe, Cr, the concentration of Fe, Cr of chemical strengthening salt aredetermined by the calibration curve after analyzing intensity of theatomic emissions of filtering solution dissolved with the chemicalstrengthening salt in solvent by a filter, dissolving particles capturedby the filter in acid, and measuring a known concentration sample inadvance.
 5. A method of inspecting chemical strengthening salt, wherein:at least one of the analyzing method claimed in claim 1 is used.
 6. Amethod as claimed in claim 1, wherein: the glass substrate for theinformation recording medium comprises a glass substrate for a magneticdisk.
 7. A method as claimed in claim 6, wherein: the glass substratefor the magnetic disk comprises a glass substrate for a magnetic diskwhich is used by combining with a magneto-resistive head or a giantmagneto-resistive head.
 8. A method of manufacturing an informationrecording medium, wherein: at least a recording layer is formed on theglass substrate obtained by the manufacturing method of the glasssubstrate for the information recording medium claimed in claim
 1. 9. Amethod of manufacturing an magnetic disk, wherein: at least a magneticlayer is formed on the glass substrate obtained by the manufacturingmethod of the glass substrate for the information recording mediumclaimed in claim
 1. 10. A method of manufacturing an optical glasssubstrate, including the step of exchanging ions in a glass componentwith ions in an ion exchange processing liquid by dipping said glasscomponent in said ion exchange processing liquid, a salt satisfying theconditions described in claim 1 is used in said ion exchange processingliquid.
 11. A method as claimed in claim 10, wherein: said optical glasssubstrate is a glass substrate for an electronic device or a glasssubstrate for an optical device and said ion exchange processing liquidis a chemical strengthening processing liquid containing a chemicalstrengthening salt.
 12. A method of inspecting chemical strengtheningsalt, wherein: at least one of the analyzing method claimed in claim 2is used.
 13. A method of inspecting chemical strengthening salt,wherein: at least one of the analyzing method claimed in claim 3 isused.
 14. A method of inspecting chemical strengthening salt, wherein:at least one of the analyzing method claimed in claim 4 is used.
 15. Amethod as claimed in claim 2, wherein: the glass substrate for theinformation recording medium comprises a glass substrate for a magneticdisk.
 16. A method as claimed in claim 3, wherein: the glass substratefor the information recording medium comprises a glass substrate for amagnetic disk.
 17. A method as claimed in claim 4, wherein: the glasssubstrate for the information recording medium comprises a glasssubstrate for a magnetic disk.
 18. A method of manufacturing aninformation recording medium, wherein: at least a recording layer isformed on the glass substrate obtained by the manufacturing method ofthe glass substrate for the information recording medium claimed inclaim
 2. 19. A method of manufacturing an information recording medium,wherein: at least a recording layer is formed on the glass substrateobtained by the manufacturing method of the glass substrate for theinformation recording medium claimed in claim
 3. 20. A method ofmanufacturing an information recording medium, wherein: at least arecording layer is formed on the glass substrate obtained by themanufacturing method of the glass substrate for the informationrecording medium claimed in claim
 4. 21. A method of manufacturing anmagnetic disk, wherein: at least a magnetic layer is formed on the glasssubstrate obtained by the manufacturing method of the glass substratefor the information recording medium claimed in claim
 2. 22. A method ofmanufacturing an magnetic disk, wherein: at least a magnetic layer isformed on the glass substrate obtained by the manufacturing method ofthe glass substrate for the information recording medium claimed inclaim
 3. 23. A method of manufacturing an magnetic disk, wherein: atleast a magnetic layer is formed on the glass substrate obtained by themanufacturing method* of the glass substrate for the informationrecording medium claimed in claim
 4. 24. A method of manufacturing anoptical glass substrate, including the step of exchanging ions in aglass component with ions in an ion exchange processing liquid bydipping said glass component in said ion exchange processing liquid, asalt satisfying the conditions described in claim 2 is used in said ionexchange processing liquid.
 25. A method of manufacturing an opticalglass substrate, including the step of exchanging ions in a glasscomponent with ions in an ion exchange processing liquid by dipping saidglass component in said ion exchange processing liquid, a saltsatisfying the conditions described in claim 3 is used in said ionexchange processing liquid.